Low loss optical switching system

ABSTRACT

An optical switching system that switches the path of an optical signal by moving a microstructure onto which a light-guiding structure is mounted. The microstructure is formed by a MEMs and semiconductor process to be integral to the substrate. The light-guiding structure may include waveguides. The microstructure moves from one position to another position (e.g., laterally, vertically, rotationally) such that incoming optical signals align over a small air gap to different optical paths, depending on the position of the movable microstructure. As a result, the optical signal propagate along different optical paths (e.g., straight pass through or cross over) depending on the position of the movable microstructure. The optical paths have a large radii of curvature so as to change the direction of the optical signal gradually, thereby reducing insertion losses. By combining optical switches in both the vertical and horizontal directions, the resulting optical switching system handles switching in three dimensions.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application is a continuation of co-pending U.S.patent application Ser. No. 10/052,829, filed on Oct. 19, 2001, which isa continuation-in-part of U.S. patent application Ser. No. 09/837,829,filed on Apr. 17, 2001, and U.S. patent application Ser. No. 09/837,817,filed on Apr. 17, 2001, the disclosures of which are incorporated hereinby reference. This application also claims priority to provisional U.S.patent application Ser. No. 60/233,672, filed on Sep. 19, 2000, andprovisional U.S. patent application Ser. No. 60/241,762, filed on Oct.20, 2000, the disclosures of which are incorporated herein by reference.This patent application is also related to U.S. patent application Ser.No. 09/046,416, filed on Oct. 19, 2001, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

[0002] The field of the invention relates generally to a device andmethod for switching an optical signal with lower bending losses and inparticular, to a device and method for switching an optical signal bygradually changing the direction of the optical signal.

BACKGROUND OF THE INVENTION

[0003] The interest in optical switching devices has been driven by thetremendous increase in demand for more usage and faster communicationssystems, i.e. greater bandwidth, in the telecommunication industry. Theprime examples of applications that are pushing this demand are theInternet, video/music on demand, and corporate data storage. Theexisting telecommunication infrastructure, which was largely developedfor telephone calls, is now incapable of meeting the demands for newapplications of data communication.

[0004] Several options have been developed to meet this new demand.These options include wireless, optical, and free-space lasercommunication technologies. To date, the most promising technologycapable of meeting the projected bandwidth requirements of the future isthe optical technology.

[0005] In an all optical network, or in a combination of an optical andelectrical network, the necessary components include a signal carriermedium (i.e. optical fiber), signal routing systems, and data controlsystems. These signal routing systems have devices which switch opticalsignals between optical fibers.

[0006] In the prior art approaches, the switching of optical signals canbe accomplished in predominantly two major approaches: electrical andoptical. Today, most systems use electrical switching. In these systems,at the network junctions, the optical signals must first be convertedinto electrical signals. The converted electrical signals are thenswitched to the designated channel by integrated circuits. Lastly, theelectrical signals must be converted back into optical signals beforethe signals can be passed onto the optical fiber toward the nextdestination. Such optical converters are relatively expensive comparedto the rest of the transmission equipment.

[0007] Electrical switching technology is reliable, inexpensive (exceptfor optical converters), and permits signal reconditioning andmonitoring. The main drawback with electrical switching systems is thatthe number of junctions in a long distance network can be large, and thetotal cost of converters is very high. Furthermore, typically more than70% of signals arriving at a junction require only simple straightpass-through, and conversion (down and up conversions) of the fullsignal results in inefficient use of hardware. System designers alsoanticipate that future systems are best served by transparent opticalswitch capabilities; that is, switching systems capable of redirectingthe path of the optical signal without regard to the bit rate, dataformat, or wavelength of the optical signal between the input and outputports. Most electrical switching systems are designed for a specificrate and format, and cannot accommodate multiple and dynamic rates andformats. Future systems will also be required to handle optical signalsof different wavelengths, which in an electrical switching network wouldnecessitate the use of separate channels for each wavelength. Theselimitations of the electrical switching system provide new opportunitiesfor the development of improved optical switching systems.

[0008] A switch that directly affects the direction of light path isoften referred to as an Optical Cross Connect (OXC). Conventionaloptical fabrication techniques using glass and other optical substratescannot generate products that meet the performance and cost requirementsfor data communication applications. Unlike the electrical switchingtechnique that is based on matured integrated circuit technology,optical switching (ones that can achieve high port count) depends ontechnologies that are relatively new. The use of micromachining is onesuch new approach. The term MEMS (Micro Electro-Mechanical Systems) isused to describe devices made using wafer fabrication process bymicromachining (mostly on silicon wafers). The batch processingcapabilities of MEMS enable the production of these devices at low costand in large volume.

[0009] MEMS-based optical switches can be largely grouped into threecategories: 1) silicon mirrors, 2) fluid switches, and 3)thermal-optical switches. Both fluid and thermal-optical switches havebeen demonstrated, but these technologies lack the ability to scale upto a high number of channels or port counts. A high port count isimportant to switch a large number of fibers efficiently at thejunctions. Thus far, the use of silicon mirrors in a three dimensional(3D) space is the only approach where a high port count (e.g., greaterthan 1000) is achievable.

[0010] Optical Cross Connects that use 3D silicon mirrors face extremechallenges. These systems require very tight angular control of the beampath and a large free space distance between reflective mirrors in orderto create a device with high port counts. The precise angular controlsrequired are typically not achievable without an active control of beampaths. Since each path has to be monitored and steered, the resultingsystem can be complex and costly. These systems also require substantialsoftware and electrical (processing) power to monitor and control theposition of each mirror. Since the mirror can be moved in two directionsthrough an infinite number of possible positions (i.e., analog motion),the resulting feedback acquisition and control system can be verycomplex, particularly for a switch having large port counts. Forexample, as described in a recent development report, LucentTechnology's relatively small 3D mirror-switching prototype wasaccompanied by support equipment that occupied three full-size cabinetsof control electronics.

[0011] Ideally, an optical switch will have at least some of thefollowing principal characteristics:

[0012] 1) Be scalable to accommodate large port counts (>1000 ports);

[0013] 2) Be reliable;

[0014] 3) Be built at a low cost;

[0015] 4) Have a low switching time;

[0016] 5) Have a low insertion loss/cross talk.

[0017] While the 3D-silicon mirror can meet the scalability requirement,it cannot achieve the rest of the objectives. Therefore, prior pendingpatent applications, U.S. patent application Ser. Nos. 09/837,829(docket 263/176) and 60/233,672, presented a new approach whereby thecomplex nature of the 3D free space optical paths and analog control canbe replaced with guided optical paths and digital (two states)switching. Such a system greatly simplifies the operation of switching,enhance reliability and performance, while significantly lowering cost.However, there is a need to further improve devices used for switchingoptical signals because in optical switches, one of the key figures ofmerit is the Insertion Loss, a parameter that measures the amount oflight lost as a result of optical signal traversing through the switch.

[0018] The insertion loss consists of a number of components, includingloss due to coupling between fiber and switch element, loss due toabsorption of light in the waveguide material, and loss due to lighttraversing in a curved path or around corners. For example, if awaveguide has high-angle bends, there are greater losses in the opticalsignal passing through the bends. In particular, there is a need toreduce the bending losses in an optical switch element while minimizingthe element size. Ideally, the improvement would minimize individuallosses and balance the losses between different mechanisms to yield thelowest total insertion loss. In addition to the insertion loss and smallelement size, other requirements such as power, switching time, andpolarization effects are also important considerations in the design.

[0019]FIG. 9 is adapted from related and copending U.S. patentapplication Ser. Nos. 09/837,829 (docket 263/176) and 60/233,672 andillustrates a concept for using movable microstructure to switchingoptical signals. In FIG. 9, waveguides 501 are used to conduct opticalsignals from input 502 to output connections 503. For additional detail,please refer to U.S. patent application Ser. No. 09/837,829. To enablelight paths to crossover, waveguide designs with approximately 90-degreebends 504 are shown in FIG. 9. Although a 90-degree bend is possible,such design must be done under numerous constraints; in particular, bendradius. For example, the optical loss due to a waveguide with a bendradius R can be estimated as:${{{Bend}\quad {Loss}} = {10\quad \log \quad {\exp \lbrack {{- ( {R\quad \Theta} )}\frac{1}{{kn}_{eff}a^{2}}\frac{U^{2}W^{2}^{2\quad W}}{1 + W}{\exp ( {{- \frac{4}{3}}\frac{W^{3}\Delta \quad R}{V^{2}a}} )}} \rbrack}\quad ( {{in}\quad {dB}} )}},$

[0020] where Δ=(n₁ ²-n₂ ²)/(2n₁ ²) is a measure of the differencebetween the refractive index of the core of the waveguide (n₁) andmaterial that surrounds the core (n₂). From the equation above, it canbe shown that when a small radius is required, it is possible tocompensate for loss by using large Δs. Materials with a wide range ofrefractive indexes have been used successfully in waveguides includingsilica, silicon, polymer and various other materials.

[0021]FIG. 10 illustrates a typical waveguide design where differentcomponents of the waveguide are identified. The same material, such assilica, is used for the core 505 as well as for the buffer 506 andcladding 507, but the core is doped with another material to increaseits index of refraction. The buffer 506 may be adjacent to a siliconsubstrate. Using a cladding 507 is not always required since air has anindex of refraction (n=1.00) that is lower than any solid material andcan be used to guide light effectively.

[0022] A main problem with employing large Δs is that the size of thewaveguide must be substantially reduced to maintain single modepropagation, which is an important criterion for telecommunicationapplications. The relationship between waveguide core width for a squarewaveguide and Δ for single mode propagation is illustrated by thefollowing equation: $d = \frac{4.272}{{kn}_{1}\sqrt{2\Delta}}$

[0023] As can be seen in above equation, the larger the Δ, the smallerthe core size d required. The problem with using small waveguides isthat it increases the optical loss due to fiber coupling with a largecore fiber. To minimize coupling loss, a lens element is required tomatch the mode between the fiber and waveguide, which leads to highermanufacturing costs.

[0024] A design capable of accommodating large bend radii whilemaintaining a small size, is highly beneficial to controlling theoverall insertion loss. A small size switch element is desirable becausemore elements can be produced on a single wafer. Small elements alsokeep the finished size small when they are used in an array connected toform a large port switch.

SUMMARY OF THE INVENTION

[0025] The invention relates generally to an optical switching device ormethod of switching an optical signal, which device or method uses amovable microstructure to switch the direction of the optical signalgradually so as to reduce insertion loss.

[0026] Other systems, methods, features and advantages of the inventionwill be or will become apparent to one with skill in the art uponexamination of the following figures and detailed description. It isintended that all such additional systems, methods, features andadvantages be included within this description, be within the scope ofthe invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The components in the figures are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention. Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views. However, somecorresponding parts may be given unique reference numerals.

[0028]FIG. 1 illustrates a block diagram of an example embodiment of anoptical switch system adapted to handle 1024 ports.

[0029]FIG. 2 illustrates an exploded conceptual view of an exampleembodiment of the OXC blocks and optical connectors of FIG. 1.

[0030]FIG. 3A illustrates a plan view of an example embodiment of asingle switching layer of FIG. 2.

[0031]FIG. 3B illustrates an edge view of an example embodiment of asingle switching layer of FIG. 2.

[0032] FIGS. 4A-4F illustrate different example embodiments of awaveguide on a switching layer.

[0033]FIG. 5A illustrates an plan view of an example embodiment of aswitching layer which can switch 8×8 ports.

[0034]FIG. 5B illustrates an edge view of the switching layer of FIG.5A.

[0035]FIG. 6A illustrates an example embodiment of an optical connectorwhose optical substrate is machined to have an array of convex sphericalsurfaces.

[0036]FIG. 6B illustrates how the optical connector of FIG. 6A correctsa misaligned light beam.

[0037]FIG. 7A illustrates an example embodiment of a switch elementhaving a movable optically transmissive platform.

[0038]FIG. 7B illustrates the switch element of FIG. 7A when the movableplatform is not moved.

[0039]FIG. 7C illustrates the switch element of FIG. 7A when the movableplatform is moved.

[0040]FIG. 7D illustrates an example embodiment of a switch elementhaving a movable optically transmissive platform and a double layer ofwaveguides.

[0041]FIG. 8A illustrates an example alternative embodiment of a switchelement having a movable optically transmissive platform which movesparallel to the plane of the substrate.

[0042]FIG. 8B illustrates an example alternative embodiment of a switchelement having a rotatable or pivoting optically transmissive platform.

[0043]FIG. 9 illustrates a block diagram of a prior optical switchdevice as described in related and co-pending U.S. patent applicationSer. Nos. 09/837,829 (docket 263/176) and 60/233,672.

[0044]FIG. 10 illustrates a typical waveguide and its structures.

[0045]FIG. 11 illustrates a block diagram of an example embodiment of animproved optical switching device having a movable microstructure withlow insertion loss.

[0046]FIG. 12 illustrates a block diagram of another example embodimentof an improved optical switching device having a rotatablemicrostructure with low insertion loss.

[0047]FIG. 13 illustrates a block diagram of an example embodiment of asystem of improved optical switching devices with movablemicrostructures and low insertion losses.

[0048]FIG. 14A illustrates a block diagram of an example embodiment ofan improved 1×2 optical switching device with a movable microstructureand low insertion losses, where the movable microstructure is in a firstposition.

[0049]FIG. 14B illustrates a block diagram of an example embodiment ofan improved 1×2 optical switching device with a movable microstructureand low insertion losses, where the movable microstructure is in asecond position.

[0050]FIG. 15 illustrates a block diagram of yet another exampleembodiment of an improved 1×2 optical switch device with a rotatablemicrostructure and low insertion losses.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] The first portion of this specification refers to FIGS. 1-8B anddiscusses an improved optical switch system. The second portion of thisspecification refers to FIGS. 9-15 and sets forth an improvement to theoptical switch system of FIGS. 1-8B, where the improvement graduallychanges the direction of an optical signal in order to reduce insertionlosses.

[0052]FIG. 1 illustrates a block diagram of an example embodiment of anoptical switch system 10 adapted to handle 1024 ports by 1024 ports.This optical switch system 10 includes a 3-dimensional waveguide. The 3Doptical switch system 10 shown in FIG. 1 employs guided wave paths(i.e., waveguide), digital switching, and is capable of handling 1024ports. Two of the key components of the optical switch system 10 are twoOXC blocks 12, 14. OXC blocks 12, 14 are also referred to as switchblocks because they include vertical and horizontal optical switchesrespectively. OXC block (Y) 12 is used for switching optical beams inthe vertical direction, and OXC block (X) 14 switches optical beams inthe horizontal direction. The two OXC blocks (Y and X) 12, 14 areconnected end-to-end such that all outputs of the first (Y) OXC block 12is connected to the input of the second (X) OXC block 14.

[0053] Since each OXC block 12, 14 is an assembled unit, somemanufacturing tolerances may be inevitable. To handle the accumulationof these tolerances, an optical connector 16 is required to facilitatesystem assembly. Likewise, optical connectors 16 may be used at theinput of the first OXC block 12, and output of the second OXC block 14,to allow for positional errors at the interface connection. Optionally,the optical connector 16 can be an optical-to-electrical-to-opticalconnector, a plurality of mirrors in free space, a bundle of opticalfibers, or any kind of optical connector.

[0054] Optical fibers 18 are connected to the input interface 20. Theswitched optical signals exit at the output interface 22. For example,the input interface 20 and output interface 22 may be mechanicalinterfaces to fiber optics. Electrical signals for controllingindividual switch elements are interconnected (between layers) in anelectrical interconnect 24 on the side of each OXC block 12, 14. Theseelectrical wires are routed to the Interface and Control Electronics 30located adjacent to the OXC blocks 12, 14. The optical switch system 10may be mounted on a board 32.

[0055]FIG. 2 illustrates an exploded conceptual view of an exampleembodiment of the OXC blocks 12, 14 and optical connectors 16A-16C ofFIG. 1. For clarity, the vertical switch block, OXC block 12, is shownwith only the first and last switching layers 40, 42. Each switchinglayer 40, 42, for example, is capable of switching 32 inputs to any ofthe 32 outputs in the vertical direction. By placing 32 of the switchinglayers together, all 32 channels can be connected along the verticalplane. To complete the full capability of switching 32×32 channels, amechanism for switching in the horizontal direction is needed and thisis fulfilled, for example, by a second OXC block 14 (the horizontalswitch block). FIG. 2 shows only the first and last switching layers 44,46 of the second (X) OXC block 14. Each switching layer 44, 46, forexample, is capable of switching 32 inputs to any of the 32 outputs inthe horizontal direction. By placing 32 of the switching layerstogether, all 32 channels can be connected along the horizontal plane.Combined into the embodiment shown in FIG. 2, the vertical andhorizontal switching layers create a 32×32 optical switch.

[0056] The following example illustrates how a signal at channel (1,1)(the numbers refer to the row and column number respectively) can berouted to the channel (32,32) output. The optical beam 50 (representedin arrows) enters at the (1,1) location, through first optical connector16A, and enters the first switching layer 40. The switches in the firstswitching layer 40 connect the optical beam from (1,1) to the (1,32)output. The optical signal exits the vertical (Y) switch layers, andpasses and realigns properly through the second optical connector 16Binto the horizontal (X) switching layer at (1, 32). The optical beam nowis routed from position (1,32) to position (32,32), then realigns andexits through the third optical connector 16C.

[0057] The optical switch system 10 may have an optical path network202, which is also referred to as a light-guiding structure. The opticalpath network 202 includes at least one optical path along which theoptical signal 50 may travel. For example, the optical path network 202may include a mirror, waveguide, air gap, or other structures thatprovide an optical path. In the example embodiment, the optical pathnetwork 202 is a waveguide network 202. One advantage of the 3Dwaveguide embodied in the optical switch system 10 described is that inthis approach it is possible to achieve a large port count without aneed to control the beam paths precisely and actively. Since the opticalbeam is captured within the waveguides or waveguide networks on eachswitching layer, only the end connections are critical. A waveguidenetwork may include a plurality of waveguides such as waveguide network202 shown in FIG. 8A. In fact, a waveguide network may contain only asingle waveguide, if desired. Where an embodiment is described as usinga waveguide network, it should be understood that the embodiment coulduse a waveguide instead, and vice versa. Where alignment is critical,such as at the interface, an optical connector 16 will allow forcorrection of beam misalignment using conventional and inexpensiveoptics. The simplicity of the resulting 3D waveguide and the protectiveenvironment (e.g., each switching layer can be sealed) further enhancesthe reliability and robustness of the system, providing beam paths whichare unaffected by temperature, humidity, aging and handling.

[0058]FIGS. 3A and 3B illustrate a plan view and an edge viewrespectively of an example embodiment of a single switching layer ofFIG. 2, for example, switching layer 44. This example shows how 32inputs can be connected through an array of simple switch elements 60,to 32 outputs. In this example of a 32×32 port, there are 80 switchelements 60. The methodology of interconnection is well known to thoseskilled in the art of signal routing design and may be any methodology.Pioneering work in routing theories done at Bell Laboratories has shownthat an optical signal can be efficiently routed by connecting simpleswitches (such as 2×2 elements) in a specific manner. By following theserouting guidelines, it can be shown that every input can be connected toany output without any of the connections blocked.

[0059] The switching layer 44 shown in FIGS. 3A, 3B includes a substrate62 that carries waveguides 64 and switch elements 60. In this exampleembodiment, the substrate 62 may be any semiconductor material such assilicon. In any embodiment, the substrate can be any kind of substrate.The substrate can be a composite layer made by bonding wafers together,or a monolithic layer. To protect these waveguide and switch elementmicrostructures, the substrate 62 may be covered and sealed by usinganother (cap) wafer 63. An effective sealing to exclude contaminants andhumidity can be achieved by bonding a cap wafer 63 to substrate 62 usingany of a multitude of techniques already available, including anodic,fusion, and eutectic bonding.

[0060] Optical signals 50 enter the switching layer 44 at one edge.Preferably, the edge is polished and angled to allow a completerefraction of the optical beams 50. Depending on the optical index ofthe interface medium (e.g., air or another optical element), the angleof the edge can be designed to accommodate total refraction. Once theoptical beam 50 enters the waveguide 64, light cannot escape from thewaveguide 64 due to a phenomenon known as total internal reflection.This is the same phenomenon that allows an optical fiber to carry lightfor long distances without significant loss.

[0061] The switching action is controlled by the application ofelectrical voltage. Each switch element 60 requires, for example, threeelectrical connections: an actuation electrode, a position sensingelectrode, and electrical ground. The electrical ground connection canbe tied together to minimize the number of electrical traces. Eachswitch element 60 would have, therefore, a minimum of two electricalconnections that need to be passed through and underneath the cappingwafer 63 to interface with the outside world. In FIG. 3A, the electricaltraces 66 are shown traversing substantially orthogonally to the opticalpath and terminating at the electrical bond pads 68 at the lower edge.Of course, the actual layout of the electrical traces 66, bond pads 68,input ports and output ports can be modified to be different than thatshown in this example.

[0062] FIGS. 4A-4F illustrates various example embodiments of awaveguide 64 on a switching layer. To maintain total internal reflection(TIR), the environment surrounding the waveguide 64 must have an opticalindex of refraction lower than index of the waveguide 64. Glass, forexample, which has an index of 1.5, can be coated with a material havinga lower index, or simply use a vacuum (index 1.0) or air as the medium.A wide range of gases could be used to ensure compatibility with thewafer bonding process. In a first embodiment, FIG. 4A illustrates across section of a waveguide 64 formed of glass whereby the mediumsurrounding the waveguide 64 is in a vacuum or air. The carrier 70 maybe formed of glass or silicon. In a second embodiment, FIG. 4Billustrates another waveguide 64 where the top and sides of thewaveguide 64 are in contact with a vacuum while the bottom surface isbonded with an intermediate material with an index lower than that ofthe waveguide. The carrier 70 may be formed of glass or silicon.

[0063] In both of the FIG. 4A and 4B embodiments, the upper substrateshould be a material that will transmit optical signals at thewavelength of interest, such as 0.82, 1.3, and 1.55 micrometers. Theseare the wavelengths that are typically used in fiber opticstransmission, and in which the support equipment (such as thetransmitter, carrier and receiver) is designed to handle. In bothembodiments, the material on the bottom (carrier substrate 70) is usedmainly to provide mechanical support to the structure. As it will beexplained later, the actual switching mechanism will require some of thewaveguides to move vertically or laterally by the application of anexternal force. The carrier substrate 70 can be made of glass, silicon,or any material compatible with micromachining.

[0064]FIGS. 4C and 4D illustrate alternative embodiments of a waveguide64 without using a substrate 70. The small amount of material 72 thatbridges the waveguide 64 to adjacent material will allow some loss oflight and this design needs to consider the tradeoff between mechanicalstrength and optical loss. One advantage of the embodiments in FIGS. 4Cand 4D is that only a single-layer structure is required, avoiding thenecessity of wafer bonding. Detailed designs using these alternativeembodiments should involve achieving a balance between the mechanicaland optical integrity of the waveguides and acceptable manufacturingcosts.

[0065] Although the preferred embodiment of an optical switch systemuses a waveguide, optical guides using reflective surfaces or otherknown structures can also be used. FIG. 4E shows a guide 78 made bybonding two wafers 80, 82 to create a closed optical guide 78. Toenhance the reflectivity of the surface, metal coating such as gold ornickel (or any other materials compatible with the micromachiningprocess) could be deposited on the inner surfaces prior to bonding.

[0066] Yet another alternative embodiment is to use the verticalsurfaces of the microstructure. As in a conventional optical system,such an approach would require tight angular control of the verticalwalls to control the beams precisely. FIG. 4F shows a trench etched intothe wafer whose vertical walls are the reflective surfaces with a topcap 80 forming a closed waveguide 78. As before, a metal coating can beapplied to enhance reflectivity.

[0067]FIGS. 5A and 5B illustrate a plan view and an edge view of anexample embodiment of a non-blocking switching layer 44 that performsswitching of 8×8 ports. To achieve full switching capability in thisexample, 12 switch elements 90 are required. Each switching element 90is capable of performing a 2×2 switch. The switching layer 44 isnon-blocking because the optical signal 50 always passes to the opticaloutput side through some optical path.

[0068] Optical connectors 16 are used to minimize insertion loss due tomisalignment between the optical fiber and the switch element 90, orbetween OXC blocks. In both cases, there is an accumulation ofgeometrical tolerances due to imperfect assembly, which should becorrected to minimize loss of light. Most often, the misalignment is dueto a combination of linear and angular offsets.

[0069]FIG. 6A illustrates an optical connector 16 whose substrate ismachined on both sides to have an array of convex spherical surfaces100. One side of the spherical surface array is positioned to connectwith a fiber bundle to receive the incoming light beam 50. The oppositeconvex surface focuses the beam onto a small spot to allow forconnection to the OXC blocks. For example, the optical connector 16 mayhave as a spherical surface 100 for each port in the optical switchingsystem (here, e.g., 32×32, or 1024 surfaces 100).

[0070]FIG. 6B illustrates how the optical connector of FIG. 6A correctsa misaligned beam of light. Let us presume a light beam 50 entering onthe left that will normally be out of the range of the entrance to theOXC block or other optical passage. If uncorrected, the light beam 50will not properly enter the entrance to the OXC block. However, themisaligned beam 50, after being corrected by a spherical surface of theoptical connector 16 will emerge from the optical connector 16 focusedon an image point 102. By placing the entrance pupil of the OXC block oroptical fiber entrance at or near the image point 102, the emerginglight beam will be approximately centered and will enter the opticalpassage such as a waveguide 64 at an incident angle that will becaptured by a total internal reflection process. Other type of surfacesother than spherical can also be used to enhance the quality of theemerged beam. The detailed design of the optical surfaces and selectionof the optical material can include those known to those skilled in theart of optical design.

[0071] The optical connector 16 which uses convex spherical surfaces 100can be manufactured using a series of spherical balls and securing thoseballs in a plate with precisely machined holes. To hold the balls inplace, the simplest method is to shrink the balls in a cold bath (e.g.,liquid nitrogen) and inserting the balls into the holes of the plate.Proper methods of fixture will allow a large number of balls to beinserted simultaneously and precisely. Alternatively, specializedtooling with convex grinding tool bits can be made to produce thedesired surfaces. The possible manufacturing techniques are numerous andinclude those well known to those skilled in the art of opticalmanufacturing.

[0072]FIG. 7A illustrates an example embodiment of a small switchelement 60 made by a micromachining process. This example embodiment isof a 2×2 switch element 60 because there are two inputs and two outputs;of course, the number of inputs and the number of outputs can beincreased or decreased. The embodiment of the switch element 60 has twowaveguides integrated on top of a carrier platform 110. The combinedstructure (waveguide and carrier) is bonded to a substrate 62 andpositioned such that the switch element 60 is suspended over an air gapover, or a cavity 111 previously etched on, the substrate 62. Thecarrier platform 110 is preferably suspended approximately 30 micronsabove the actuation electrodes 112. The carrier platform 110 movesrelative to the substrate. The waveguides 114, 116 are typically lessthan 10 microns and in this example, the small channel size is necessaryto ensure transmission of only single-mode optical signals. The size ofthe structure and the design of the support springs 130 depend on thetype of actuation mechanism used. The embodiment will use electrostaticattraction as the means of actuation.

[0073] For electrostatic actuation, both the carrier platform 110 andthe stationary electrodes 112, 126 have to be electrically conductive,thereby causing the carrier 110 to move toward the electrodes 112, 126,as illustrated in FIGS. 7B and 7C. If the carrier platform 110 is madeout of dielectric materials, it can be made conductive by coating thebottom (i.e., the surface facing the stationary actuation electrode 112)with a metal such as gold or nickel. If the carrier platform 110 is madeof semiconductor materials such as silicon, it can be doped to increaseelectrical conductivity. Opposing and parallel to the carrier platform110 are the stationary electrodes 112, 126 patterned on the bottom ofthe cavity 111. These electrodes 112, 126 connect to the top of thesubstrate 62 by traces patterned on the sloped surfaces. In the cavity111, two stationary electrodes 112, 126 are made, one electrode 112 foractuating movement of the carrier platform 110 and the other electrode126 for feedback sensing of the position of the carrier platform 110.

[0074] This example embodiment of the switch element 60 operates asfollows. Optical signals 50 enter on the left of the switch element 60at locations A and B. The optical signals 50 enter the waveguides 114,116 and cross over due to the particular configuration of the waveguidesin this embodiment. The optical signals 50 from locations A and B exitthe switch element 60 at locations D and C respectively. The originaloptical signals 50 have crossed from A to D and from B to C. When nocrossing of the optical signals 50 is desired in this particularembodiment, an electrical signal is required from the control hardware.By applying a voltage to the fixed electrodes 112 on the substrate 62and a different voltage to the electrode of the carrier platform 110,the voltage difference will result in an electrostatic attraction force.Such a force will pull the carrier platform 110 (and the waveguides 114,116 carried by the carrier platform 110) down (here, less than 10micrometers) toward the fixed electrodes 112, 126 by bending the supportsprings 130, and therefore, in the process remove the waveguides 114,116 from the optical path. The optical signals 50 from location A thenpass directly (through free space 120) toward point C, and the opticalsignals 50 from location B pass directly (through free space 122) tolocation D. FIG. 7B illustrates the case where the carrier platform 110is in its rest state because no power is applied to the actuationelectrode 112; here, the optical signals 50 from locations A and B ofthe fixed waveguides at the input side of the carrier platform 110 crossover in movable waveguides 114, 116 to locations D and C, respectively,of the fixed waveguides at the output side of the carrier platform 110;waveguides 114, 116 are considered “movable” because they move with themovement of the carrier platform 110. The carrier platform 110 is alsoreferred to as a movable microstructure. When power is applied to theactuation electrode 112, FIG. 7C illustrates the resulting configurationwhere the carrier platform 110 has moved toward actuation electrode 112;here, the optical signals 50 from locations A and B of the fixedwaveguides at the input side of the carrier platform 110 pass directlythrough free space to locations C and D, respectively, of the fixedwaveguides at the output side of the carrier platform 110 becausemovable waveguides 114, 116 have moved out of range of the opticalsignals 50.

[0075] Other methods of actuation are also viable. Electrostaticactuation is preferred because of the simplicity in design andoperation. The main drawback is the higher voltage required to operatethe resulting device, due to the large gap, typically ranging from 20 to100 volts. Alternative actuation methods include magnetic and thermaltechniques. These methods are well known to those skilled in the art ofmicromachine design.

[0076] The sensing electrode 126 on the substrate 62 is used to detectthe position of the carrier platform 110 by sensing changes incapacitance between the electrode 126 and the electrode of the carrierplatform 110 due to changes in the gap caused by movement of the carrierplatform 110. Other means of sensing, such as piezo-resistive, magnetic,optical schemes are also viable. The signal from the sensing electrode126 is used (through close-loop control) to accurately position thewaveguides 114, 116 over the optical entrance and exit.

[0077] The primary loss of optical signal will be at the entrance of themovable waveguides 114, 116 (on the carrier platform 110 of the switchelement 60) and at the entrance of the fixed waveguides. Reducing thedistance between the locations A/C and between B/D can minimize suchloss. To fully minimize loss, but with increased manufacturingcomplexity, a secondary waveguide 138, 140 can be designed on the bottomof the carrier platform 110. In that case, the opening between thestationary waveguides and the movable waveguides 114, 116 can be reducedto less than 2 microns, depending on the etching process. FIG. 7Dillustrates a carrier platform 110 with waveguides 114, 116 on top andwaveguides 138, 140 on the bottom, with one set designed for straightpass and the other for crossover. As is apparent from the embodimentshown in FIG. 7D, in the case where the carrier platform 110 is in itsrest state because no power is applied to the actuation electrode 112,the optical signals 50 from locations A and B of the fixed waveguides atthe input side of the carrier platform 110 pass through movablewaveguides 138, 140 to the fixed waveguides at the output side of thecarrier platform 110. Likewise, when power is applied to the actuationelectrode 112, the carrier platform 110 moves toward actuation electrode112 so the optical signals 50 from locations A and B of the fixedwaveguides at the input side of the carrier platform 110 now passthrough waveguides 114, 116 of the fixed waveguides at the output sideof the carrier platform 110 because movable waveguides 138, 140 havemoved out of range of the optical signals 50 and movable waveguides 114,116 have moved into range of the optical signals 50. Of course, in anembodiment which uses double movable waveguides, such as thatillustrated in FIG. 7D, the default can be either straight pass orcrossover. In other words, waveguides 114, 116 can permit a straightpass while waveguides 138, 140 causes a cross over, or vice versa.

[0078] An alternative embodiment of a MEMS switch element 60 is nowdescribed. The movement of the switch element 60 is not limited to thosein the vertical direction perpendicular to the substrate 62. FIG. 8Aillustrates an example alternative embodiment of a MEMS switch elementwhereby the actuation direction is lateral or substantially parallel tothe plane of substrate 62. FIG. 8B illustrates an example alternativeembodiment of a MEMS switch element which relies on rotational movement.Of course, an optical switching system 10 may be created from opticalswitch elements which all move in the same manner (e.g., all movevertically, all move laterally, or all move rotationally) or opticalswitch elements which move in different manners (e.g., some movevertically and others move laterally, or some move vertically and othersmove rotationally, or some move laterally and others move rotationally).The lateral movement can be induced by applying different voltages tothe inter-digitated (known as comb fingers in MEMS) structures as shownin FIG. 8A. Describing what is illustrated in FIG. 8A, the MEMS switchelement 60 comprises a substrate 62. Suspended above substrate 62, forexample over a cavity or otherwise, is a movable optically transmissiveplatform 110. Platform 110 is stated to be “optically transmissive”because it has structures (e.g., waveguide networks 200, 202) whichtransmits optical signals or light beams 50; it is not intended to meanthat the entire platform itself must be optically transmissive. One sideof the platform 110 is coupled to support springs 130 and the oppositeends of the support springs 130 are coupled or anchored to the substrate62. The platform 110 has electrodes 204. In this example, electrodes 204are inter-digitated with actuation electrodes 112. By applying differentvoltages to the electrodes 204 and actuation electrodes 112 on one sideof the platform 110 as compared to the other side of the platform 110,the platform 110 moves in a lateral, or substantially parallel, mannerrelative to the plane of the substrate 62. In FIG. 8A, this lateralmovement means that the platform 110 moves up or down.

[0079] The platform 110 carries waveguide networks 200, 202 where theoptical paths from the input side of optical signals 50 to the outputside change depending on the lateral position of the platform 110. Forexample, if the platform is in a first position (e.g., a rest position),the alignment of the incoming optical signals 50 to the inputs A, B, Cand D of the waveguide networks 200, 202 is selected such that opticalsignals 50 enter inputs C and D. Because of the particular configurationof this example of the waveguide networks 200, 202, optical signals 50which enter inputs C and D of the waveguide networks 200, 202 cross overand exit at outputs H and F respectively. If the platform 110 is thenmoved to its second position, incoming optical signals 50 would enterinputs A and B, and pass straight through to outputs E and Grespectively. Of course, the waveguide networks 200, 202 can be swappedso that the default is a straight pass through. The waveguide networksmay be configured in any shape or form to accomplish whatever opticalpaths are desired.

[0080] The lateral movement approach as shown in FIG. 8A has theadvantage of not requiring the bottom electrodes, thus reducing severalsteps in the manufacturing process. The disadvantage is that the amountof electrode area is limited due to the short height of the resultingstructure, and as a result, a large number of comb fingers may berequired to generate a sufficient attraction force. A significantlylarger electrode area may be required to operate the laterally-movingswitch element of FIG. 8A than the vertically-moving switch element ofFIG. 7A.

[0081] Turning to FIG. 8B, the movable optically transmissive platform110 moves in a rotational or pivoting fashion relative to the substrate.To accomplish rotational movement in a switch element 60, the sameelectrostatic attraction forces as used in the preferred embodimentswill work. For sensing the position of the platform 110, similarcapacitance detection techniques described in the preferred embodimentswill apply. As illustrated, this example embodiment of a rotatingplatform 110 causes inputs A and B to align with the optical signalswhen the platform 110 is in a first position. When the platform 110rotates to its second position, inputs C and D are now aligned with theoptical signals. As with all of the embodiments, the waveguides andwaveguide networks may be configured in any desired shape to achieve thedesired optical paths.

[0082] Now, we turn to FIGS. 11-15. The improvement to the opticalswitching device disclosed in this application shows several approachesto reducing the bend loss by using a large bend radius while maintainingthe element size to a minimum. Several embodiments will be discussed. Inaddition to keeping the element size small, it is also desirable to keepthe motion of the switch element to a single degree of freedom, i.e.,motion along one direction only. A single degree of freedom motiongreatly simplifies the design of the support and actuators.

[0083]FIG. 11 illustrates a block diagram of an example embodiment of animproved optical switching device having a movable microstructure withlow insertion loss. The improved optical switching element is capable ofswitching two inputs to two outputs (such a switch is referred to as an2×2 switch). The design allows a single degree-of-freedom motion (see508) while maintaining a large bend radius 509. The improved opticalswitching device operates in two positions in this example embodiment.In position one, the two input optical signals are connected straightthrough input ports A, B to designated output ports C, D respectively;in position two, the input signals entering ports A, B cross over eachother before being output at ports D, C respectively. In this exampleembodiment, all routings are done with waveguides 510 placed over themovable platform 511. Input waveguides 512, 513 and output waveguides514, 515 are placed over raised platforms 516 that are stationary(fixed) to the substrate 517.

[0084] The improved optical switching device in FIG. 11 operates asfollows. Optical signals are connected to the stationary waveguides byinput ports A (518) and B (519). The movable microstructure 511 as shownin FIG. 11 is in the first position. In this first position, two opticalsignals traverse across separate gaps 520, 521 and enter into themovable microstructure 511. On the microstructure 511, the signals enterinput ports A, B, cross over each other in waveguides 533, 534, and exitby traversing across a second set of gaps 522, 523 into the stationarywaveguides 514, 515, and exit output ports D, C respectively. To allowthe signals to go straight through (without cross over), the movablemicrostructure 511 is moved in the X direction to a second position. Inthe second position, waveguides 535, 536 are arranged such that theinput optical signals will pass straight through from the input ports A,B to the output ports C, D respectively.

[0085] The movable microstructure of any embodiment can be made ofsilicon and other microstructure materials such as quartz, ceramic,metal and alloys. Preferably, the movable microstructure is manufacturedonto the substrate by using a photo lithography process, depositing amaterial such as a semiconductor or dielectric or metal material,etching portions of material away, and repeating any of thesesemiconductor processing steps as needed. A semiconductor process or amicro-machining MEMS (Micro Electro Mechanical Systems) process, forexample, may be used to create the movable microstructure to be integralwith the substrate. The term “integral” as used in this patentapplication and claims refers to two structures that are coupledtogether by a semiconductor process. For example, if X is attached to Yby screws or bolts, X is not “integral” with Y. Further, the term“integral” does not require the two structures to be formed out ofmonolithic materials; two structures can be deemed integral to eachother if the structures are formed out of composite or multiplematerials, as well as if the structures are formed out of monolithicmaterials. For example, X can be integral with Y even if X is a platformcoupled to a device layer which has been formed on a substrate by asemiconductor process. Lastly, X can be integral with Y even if X issilicon with a doped material and Y is silicon doped differently as longas the silicon are coupled together by a semiconductor process. Thesemiconductor process includes those which bond the movablemicrostructure to the substrate.

[0086] The movable microstructure 511 is suspended over an air gap abovethe substrate 517 and is supported by springs 524, 525. The springs 524,525 are preferably made of the same material as the movablemicrostructure 511. The springs 524, 525 are connected to the substrate517 through the anchors 526. The movable microstructure 511 is connectedto a set of electrodes 527, 528 (illustrated as being shaped like combs)and matched to an opposing set of electrodes 529 fixed to the substrate517. When an electrical voltage is applied across the two electrodes,the voltage differential generates an electrostatic attraction force,causing the movable microstructure 511 to move. The springs 524, 525will deflect to move the movable microstructure 511 to the desiredposition. The use of electrostatic actuators to move a microstructure iswell known to those skilled in the art of MEMS design. The waveguides512-515 are deposited on top of the stationary platform 516 and themovable microstructure 511 using standard waveguide manufacturingprocesses.

[0087] To enable the waveguides to efficiently conduct light across thegaps, the waveguides 533-536 on the movable microstructure 511 must bealigned accurately to the fixed waveguides 512-515. This can beaccomplished in two ways: by having mechanical stops or by electronicsposition control. Mechanical stops can be placed adjacent to the movablemicrostructure 511 and located at the desired distance from the movablemicrostructure 511. If there are only two positions, two stops would berequired. The achievable alignment accuracy is dependent on the accuracyof the etching process.

[0088] In FIG. 11, a set of electrodes 530 are used for sensing theposition of the movable microstructure 511. The sensing electrodes 530,similar to the actuator electrodes 527, 528 are preferably arrangedusing the comb-like structures. As the movable microstructure 511 moves,the capacitance across the comb-like electrodes 530 changes, which canbe measured using appropriate detection circuits. The movablemicrostructure 511 can be positioned accurately based on the measuredcapacitance signal. For high reliability, the signals from the sensingcircuit can also be fed into a closed-loop control circuit such that themovable microstructure 511 can be driven accurately into the desiredposition. The electrodes 530, 527, 528 are routed to the edge of thesubstrate 517 for connection to wire bond pads 531, 532. The sensingcircuits and detailed electronic designs are well known to those skilledin design of MEMS structures.

[0089] As can be seen in FIG. 11, the waveguides 533-536 on the movablemicrostructure 511 have a large bend radius (see, e.g., 509, 537). Thislarge bend radius 509 gradually changes the direction of the opticalsignal contained within the waveguide, thereby reducing the insertionloss. Preferably, the waveguides 533-536 on the movable microstructure511 are made up of one or more short waveguide portions, each of whichhaving a large bend radius to gradually change the direction of theoptical signal. The movable microstructure 511 in FIG. 11 extends in theX and Y directions. The X and Y axes are illustrated in FIG. 11 forconvenience. In the example embodiment of FIG. 11, the movablemicrostructure 511 moves in the X direction (also denoted by referencenumeral 508).

[0090] In FIG. 11, a notch 538 in the movable microstructure 511improves the ability of the waveguides 533-536 to gradually change theoptical path of the optical signal. The notch 538 and its operation arenow described for the example embodiment illustrated in FIG. 11. Thenotch 538 is in the edge of movable microstructure 511 which issubstantially along the Y axis. The notch 538 has an edge 539 which issubstantially parallel to the Y axis and an edge 540 which issubstantially parallel to the X axis. When the movable microstructure511 is in the first position, stationary waveguides 512, 513 are alignedwith movable waveguides 533 and 534, respectively, where stationarywaveguides 512, 513 meet (via small air gaps 521, 520) movablewaveguides 533 and 534, respectively, at the edge 540 of the notch 538.The optical signals cross over in movable waveguides 533 and 534 andexit the movable waveguides 533 and 534 at an edge substantiallyparallel to the X axis of the movable microstructure 511, where theoptical signals enter stationary waveguides 515, 514, respectively. Whenthe movable microstructure 511 is moved to the second position,stationary waveguides 512, 513 are aligned with movable waveguides 535and 536, respectively, where stationary waveguides 512, 513 now meetmovable waveguides 535 and 536, respectively, (via small air gaps 521,520) at the edge 540 of the notch 538. The optical signals pass through(with no crossover) in movable waveguides 535 and 536 and exit themovable waveguides 535 and 536 at an edge substantially parallel to theX axis of the movable microstructure 511, where the optical signalsenter stationary waveguides 514, 515, respectively. The presence of thenotch 538 allows the improved optical switching device to change thedirection of the optical signal even more gradually. As a result, theedge 540 of the notch 538 (into which the optical signal enters) issubstantially parallel in FIG. 11 to the edge of the movablemicrostructure 511 out of which the optical signal exits. Of course, theshapes and sizes of the perimeter of the notch 538 and of the edges ofthe movable microstructure 511 may be changed to other suitable shapesand sizes.

[0091]FIG. 12 illustrates a block diagram of another example embodimentof a low loss improved optical switching device where the movablemicrostructure 543 is a rotatable microstructure. For convenience, theterms “movable microstructure” and “movable platform” also refer to anymoving microstructure, including those which rotate and those which movelinearly. In this configuration, the motion of the movablemicrostructure 543 still has a single-degree of freedom, but thesingle-degree of freedom is angular rather than linear. The movable orrotatable microstructure 543, in this particular example embodiment, isin the shape of a ring. There are four leaf-like waveguide structures545 inside the ring. Preferably, the leaf-like waveguide structures havelarge radii of curvature so that the waveguides change the direction ofthe optical signal gradually. Outside the ring are the electrodes usedfor actuation 546 and sensing 547. The movable waveguides 548 arelocated on top of the movable microstructure 543, with connection pointslocated at four locations (549, 550, 551, 552). The ring structure isconnected to the springs 553, and the entire movable microstructure 543is suspended over an air gap above the substrate. The springs 553 areconnected to the substrate through the anchors 554. Input and outputsignals are connected to the movable microstructure 543 throughstationary waveguides 450, 452, 454 and 560. The stationary waveguides450, 452, 454 and 560 are preferably located on four raised platforms555, 556, 557, 558, respectively, so that the waveguides are all at thesame height.

[0092] The optical switching device of FIG. 12 operates as follows.Incoming optical signals are connected to the top and left side of theswitch. As shown in FIG. 12, when the movable microstructure 543 is inthe first position, the stationary waveguide 450 at input ports A (549)is coupled (preferably via a small air gap) to movable waveguide 548 inorder to route a first optical signal to stationary waveguide 454 atoutput port C (551). Similarly, the stationary waveguide 452 at inputport B (550) is coupled (preferably via a small air gap) to movablewaveguide 456 in order to route a second optical signal to cross overthe first optical signal, where the second optical signal passes throughstationary waveguide 560 and exits output port D (552). In allembodiments, the waveguides are preferably coupled to one another bysmall air gaps. Smaller air gaps permit better alignment betweenwaveguides.

[0093] When the movable microstructure 543 is moved to the secondposition, the stationary waveguide 450 at input ports A (549) is nowcoupled (preferably via a small air gap) to movable waveguide 458 inorder to route the first optical signal to stationary waveguide 560 atoutput port D (552). Similarly, in the second position, the stationarywaveguide 452 at input port B (550) is coupled (preferably via a smallair gap) to movable waveguide 460 in order to route a second opticalsignal to cross over the first optical signal, where the second opticalsignal passes through stationary waveguide 454 and exits output port C(551). Thus, when the movable microstructure 543 is in the firstposition, an optical signal from input port A (549) will exit outputport C (551) and an optical signal from input port B (550) will exitoutput port D (552). By contrast, when the movable microstructure 543 isin the second position, an optical signal from input port A (549) willexit output port D (552) and an optical signal from input port B (550)will exit output port C (551).

[0094] To rotate the movable microstructure 543 between the first andsecond positions, a differential voltage is applied to the driveelectrode 546, which is fixed to the substrate, and the movableelectrode 559 attached to the ring. An attractive force is generatedbetween the fixed electrode 546 and the moveable electrode 559 as aresult of the voltage differential, which rotates the ring in aclockwise direction. To maximize the coupling of light between thestationary and movable waveguides, the angular position of the ringshould be precisely controlled. This control can be achieved bymonitoring the change in capacitance from the sensing electrodes 547. Anelectrical circuit that converts the change in capacitance to voltagewill be required and is commercially available. Alternatively,mechanical stops could also be used to position the ring structureaccurately. The design of comb electrodes and associated sensingcircuits are well known to those skilled in the art of MEMS design.

[0095] As with the example embodiment illustrated in FIG. 11, theexample embodiment of FIG. 12 also gradually changes the direction ofoptical signals, thereby reducing insertion losses. Large bend radii arealso used. Of course, the shapes and sizes of the ring, waveguides andedges of the movable microstructure 511 may be changed to other suitableshapes and sizes.

[0096] The optical switching devices discussed above represent a basicbuilding block that can be used to build larger-port switches. Eachoptical switching device is capable of switching two ports so an arrayof these optical switching devices can be combined to switch 4, 8, 16,32, or more ports. Based on the optical switching devices of FIG. 11,FIG. 13 shows an array of six optical switching devices 561 on asubstrate 569, forming a single switch capable of connecting four inputports (562) to any of the four output ports (563). Electrical traces 567connect the optical switching devices' actuator and sensor to the wirebond pads 568 located on edge of the substrate 569.

[0097] There are a number of different architectures that use the basic2 by 2 or 1 by 2 switches to form larger switches. The example shown inFIG. 13 is based on a Benes architecture. Other popular architecturesinclude crossbar, Spanke and Clos networks. These multi-stage networksare well known to those skilled in the art of network switch designs.The design of the network and the routing paths should also be carefullyconducted to minimize optical loss. In FIG. 13, for example, the largestpossible radius should be used for the waveguides 564 between adjacentoptical switch devices 565, 566 in order to minimize loss.

[0098] Of course, the optical switching devices with rotatablemicrostructures of FIG. 12 can also be used to create an array ofmultiple optical switching devices, forming a single switch capable ofconnecting four input ports to any of the four output ports.

1×2 and 1×N Linear Switches

[0099] The techniques described herein have been illustrated for theexample of switching two optical inputs to two outputs. The same conceptmay be adapted for switching n optical signals to m output ports. Forexample, FIGS. 14A and 14B illustrate a concept of a 1×2 opticalswitching device employing linear movement. The 1×2 optical switchingdevice receives one optical signal and directs the signal to one of twooutput ports. An optical signal is transmitted into an input port A andinto a stationary waveguide 580. The optical signal propagates intoanother waveguide 583 located on top of a movable microstructure 581.The stationary waveguide 580 is located on a raised pedestal or platform582 so that the waveguide 580 is at the same height level as thewaveguides 583, 584 on the movable microstructure 581. The movablemicrostructure 581 is suspended above an air gap over the substrate 585and supported by springs 586. The springs 586 are in turn connected tothe substrate 585 by anchors 587. As shown in FIG. 14A, the opticalsignal will travel from the input waveguide 580, through the movablewaveguide 583, into stationary output waveguide 588, and exit outputport B.

[0100] To switch the optical signal to exit from another output port C,a voltage is applied to the electrodes 589, 590 as shown in FIG. 14B.The resulting electrostatic attraction will force the movablemicrostructure 581 to change its position. In this new position, thestationary waveguide 580 is coupled to the second movable waveguide 584,as well as to the stationary output waveguide 592. In doing so, thesignal is diverted into the second waveguide 584 and exits output portC. In order to set the position of the movable waveguides 583, 584precisely, electrode 591 can be used for capacitance sensing. The designof the comb actuators and sensors are similar to the details discussedabove.

[0101] The same technique can be expanded into a 1×N type of switchwhere N can be two or any number of ports. This type of switch is usefulfor connecting an optical input to a multiple number of outputs. Theswitch can also be used in reverse to connect a plurality of inputs intoone output. The following reference illustrates an application for using1×8 switch in a Broadcast and Select architecture: Jean-Paul Faure,Ludovic Noire, “An 8×8 all optical space switch based on a novel 8×1MOEMS switching module”, OFC2001, Section WX5-2, Anaheim, Calif., Mar.21, 2001.

1×2 and 1×N Rotary Switches

[0102]FIG. 15 illustrates another example embodiment of a 1×2 opticalswitching device, whereby the switching action is rotational rather thanlinear. The operation of the 1×2 optical switching device is similar tothe 2×2 optical switching devices described herein. In FIG. 15, only onestationary input waveguide 681 is used to guide an optical signal intothe movable waveguides 682 or 683. Depending on the position of themovable microstructure 684, the optical signal can be switched to any ofthe two outputs B or C.

[0103] The operation is as follows. An optical signal 680 enters inputport A, travels in stationary input waveguide 681 which is positioned ona raised platform 650, and propagates to one of waveguides 682 or 683located on the movable microstructure 684 (preferably via a small airgap between waveguide 681 and waveguide 682 or 683). In the position asshown in FIG. 15, the optical signal enters into waveguide 683,propagates into a stationary output waveguide 685 (preferably over asmall air gap) which is located on a raised platform 652, and exits theoutput port B. If a voltage differential is applied to the fixedelectrode 686 and movable electrode 687, then waveguide 682 is movedinto alignment with the input waveguide 681. In this mode, the opticalsignal propagates into waveguide 682, stationary output waveguide 687which is located on a raised platform 654, and output port C. The raisedplatforms 650, 652 and 654 are optional and serve to align thestationary waveguides 681, 685 and 687 to be at the same height as themovable waveguides 682, 683.

[0104] A cross-over 688 is shown between waveguides 682 and 683. Tominimize the optical loss associated with a cross-over, the waveguides682 and 683 are preferably located at about 90 degrees relative to eachother. Electrodes 689 can be used to determine the position of themovable microstructure 684 by connecting the electrodes to a detectioncircuit. The method of actuation and position sensing are similar to thedetails of those discussed herein. The rotational 1×2 optical switch asdescribed can also be reconfigured to a 1×N port optical switch, where Nis a number larger than two.

[0105] While various embodiments of the application have been described,it will be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof the subject invention. For example, each feature of one embodimentcan be mixed and matched with other features shown in other embodiments.Features known to those of ordinary skill in the art of optics maysimilarly be incorporated as desired. Additionally and obviously,features may be added or subtracted as desired and thus, a movableplatform having more than two sets of optical paths is alsocontemplated, whereby the platform moves to any one of three or morepositions such that each position activates a different set of opticalpaths. As another example, the optical switch may accept more than 2inputs and provide more than 2 outputs. The optical switch may becombined so as to create bigger optical switches with more ports, asdesired. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

What is claimed is:
 1. An apparatus for switching an optical signal, theapparatus comprising: a substrate; a movable microstructure formed by asemiconductor process on the substrate, the movable microstructure beingsuspended at a distance from the substrate and being adapted to moverelative to the substrate; an actuator to cause the movablemicrostructure to move from a first position to a second positionrelative to the substrate; a mirrorless light-guiding structure mountedto the movable microstructure such that the mirrorless light-guidingstructure moves with the movable microstructure, the mirrorlesslight-guiding structure including a first set of optical paths and asecond set of optical paths such that when the movable microstructure isin the first position, the optical signal travels along the first set ofoptical paths in the light-guiding structure, and when the movablemicrostructure is in the second position, the optical signal travelsalong the second set of optical paths in the mirrorless light-guidingstructure; and an input stationary waveguide coupled to the substrateand positioned to transmit the optical signal over a gap between theinput stationary waveguide and the first set of optical paths in themirrorless light-guiding structure, wherein the gap is oriented at anoblique angle to the input stationary waveguide and to the first set ofoptical paths of the mirrorless light-guiding structure.
 2. Theapparatus of claim 1, wherein the first set of optical paths has a largeradius of curvature which gradually changes the direction of the opticalsignal.
 3. The apparatus of claim 1, further comprising a notch in afirst edge portion of the movable microstructure, the first edge portionextending in a Y direction, the microstructure having a second edgeportion which extends in an X direction, the X and Y directions beingsubstantially perpendicular to each other, the notch having a third edgeportion and a fourth edge portion, wherein the fourth edge portionextends substantially parallel to the X direction, and wherein theoptical signal enters the first set of optical paths at the fourth edgeportion of the notch, wherein the gap is located adjacent to the fourthedge portion of the notch.
 4. The apparatus of claim 3 wherein theoptical signal exits the first set of optical paths at the second edgeportion of the movable microstructure.
 5. The apparatus of claim 3,wherein the movable microstructure is adapted to move in the X directionrelative to the substrate.
 6. The apparatus of claim 1, wherein the gapis an air gap.
 7. The apparatus of claim 1, wherein the mirrorlesslight-guiding structure includes a plurality of waveguides.
 8. Theapparatus of claim 7, wherein the plurality of waveguides include afirst waveguide to provide the first set of optical paths and a secondwaveguide to provide the second set of optical paths, wherein when themovable microstructure is in the first position, the first waveguide isaligned to receive the optical signal and when the movablemicrostructure is in the second position, the second waveguide isaligned to receive the optical signal.
 9. The apparatus of claim 1,further comprising an output stationary waveguide coupled to thesubstrate and positioned to receive the optical signal from the first orsecond sets of optical paths over a second gap between the substrate andthe movable microstructure, wherein the second gap is oriented at anoblique angle to the output stationary waveguide and to the first set ofoptical paths of the mirrorless light-guiding structure.
 10. Theapparatus of claim 1, wherein the width of the gap is optimize to allowboth movement of the movable microstructure and transmission of theoptical signal.
 11. An optical switching system comprising: (a) an inputport comprising a stationary input light guiding structure adapted toreceive a first optical signal, the stationary input light guidingstructure being aligned to transmit the first optical signal to one of aplurality of optical switching devices; (b) a plurality of output ports;and (c) the plurality of optical switching devices coupled to the inputport and adapted to receive the first optical signal from the stationaryinput light guiding structure and switch the first optical signal to oneof the plurality of output ports, each optical switching devicecomprising: (i) a substrate; (ii) a movable microstructure formed by asemiconductor process on the substrate, the movable microstructure beingsuspended at a distance from the substrate and being adapted to moverelative to the substrate; (iii) an actuator adapted to cause themovable microstructure to move from a first position to a secondposition relative to the substrate; (iv) a light-guiding structuremounted to the movable microstructure such that the light-guidingstructure moves with the movable microstructure, wherein thelight-guiding structure comprises a first set of optical paths and asecond set of optical paths, such that when the movable microstructureis in a first position, the first optical signal travels along the firstset of optical paths in the light-guiding structure, and when themovable microstructure is in a second position, the first optical signaltravels along the second set of optical paths in the light-guidingstructure; and (v) a gap formed between and defined by a first face ofthe stationary input light guiding structure and a second face of thefirst set of optical paths of the movable microstructure; wherein thegap causes the first optical signal to exit the stationary input lightguiding structure at an oblique angle and to enter the first set ofoptical paths at an oblique angle.
 12. The optical switching system ofclaim 11, further comprising a stationary output light guiding structurealigned to receive the first optical signal from one of the plurality ofoptical switching devices over a second gap located between the outputport and the movable microstructure, wherein the second gap is orientedat an oblique angle to the first set of optical paths and to thestationary output light guiding structure.
 13. The optical switchingsystem of claim 11, further comprising a notch in a first edge portionof the movable microstructure, the first edge portion extending in a Ydirection, the microstructure having a second edge portion which extendsin an X direction, the X and Y directions being substantiallyperpendicular to each other, the notch having a third edge portion and afourth edge portion, wherein the fourth edge portion extendssubstantially parallel to the X direction, and wherein the first opticalsignal enters the first set of optical paths at the fourth edge portionof the notch, wherein the gap is located adjacent to the fourth edgeportion of the notch.
 14. The optical switching system of claim 11,wherein the first set of optical paths has a large radius of curvaturewhich gradually changes the direction of the first optical signal. 15.The optical switching system of claim 13, wherein the first opticalsignal exits the first set of optical paths at the second edge portionof the movable microstructure.
 16. The optical switching system of claim11, wherein the gap is an air gap.
 17. The optical switching system ofclaim 11, wherein the width of the gap is optimized to allow bothmovement of the movable microstructure and transmission of the firstoptical signal.